An example of a radius end mill that is used in the machining of a work piece in which a corner cutting edge, in which an end cutting edge and a peripheral cutting edge intersect, is formed in a convex arc shape is disclosed in Japanese Unexamined Patent Application, First Publication No. S59-175915.
As is shown in FIG. 31, in this radius end mill there is provided an end mill in which end cutting edges 2 are positioned at a distal end of a tool body 1 and peripheral cutting edges 3 are positioned at a predetermined twist angle θ1 on an outer circumference of the tool body 1. In this end mill, a twist angle θ2 of corner cutting edges 4 in the vicinity of a corner of the edge tip is smaller than the twist angle θ1 of the peripheral cutting edges 3 that are connected to the corner cutting edges 4. In addition, a corner R is provided on the corner cutting edges 4. In this type of radius end mill, because the small twist angle θ2 is provided in the vicinity of a distal end of the corner cutting edges 4, the edge tip corner does not form an extremely acute angle, and working of the corner R is simplified while accuracy may be maintained. In addition, there are no defects in the edge due to the thinness of the edge tip corner portion. Moreover, because the portions of the peripheral cutting edges 3, which are the centers of the machining, are provided with a large twist angle θ1, which has excellent machining properties, materials that are difficult to machine such as titanium alloys and stainless steel may be machined easily and with a high degree of accuracy. Furthermore, it is possible to achieve a marked improvement in reducing tool costs and in the processing efficiency of the milling tasks.
However, in this radius end mill, because the twist angle θ2 of the corner cutting edges 4, which are provided with a corner R, on the distal end side of the peripheral cutting edges 3 is made weak, namely, because the rake angle in the axial direction of the corner cutting edges 4 and the end cutting edges 2 that are connected to the corner cutting edges 4 and extend to the inner circumferential side is small, although the included angle of the end cutting edges 2 and corner cutting edges 4 may be enlarged and it is possible to prevent defects, as is described above, it is not possible to prevent the blunt. Whereas, if, for example, the depth of a cut is shallow, and the center of the cut is not on the peripheral cutting edge 3 side but is on the end cutting edge 2 side, then because the distance from the center axis O of the tool body 1 is short on the inner peripheral side of the end cutting edges 2, the cutting speed is slow. Accordingly, the load during the cutting is increased and greater edge tip strength is required. In contrast, because the cutting speed is fast at the corner cutting edges 4 on the outer peripheral side of the end cutting edges 2, the load during the cutting is light, and, instead of greater edge tip strength, what is required is a sharp cutting edge. However, in a radius end mill in which the axial direction rake angle is small extending from the end cutting edges 2 to the corner cutting edges 4, in the manner described above, on the contrary, there is a possibility that there will be an increase in the cutting resistance.
Moreover, in particular, if a slanted metal surface or curved metal surface is cut using this type of radius end mill, because a number of the corner cutting edges 4 that are provided with the corners R in the vicinity of the edge tip corners thereof are used, if the sharpness of the edges in portion such as this is poor and there is considerable cutting resistance, then there is no possibility of achieving an improvement in the processing efficiency. Furthermore, in the above described conventional radius end mill, because the peripheral cutting edges 3 that are connected to the corner cutting edges 4 are provided with twist angle gradual increase portions 5 that extend from the twist angle θ2 to the large fixed twist angle θ1 so that the twist angle is made to change gradually, and because, in conjunction with this, the rake faces that are continuous with the cutting edges 4 are also formed as smoothly continuous faces whose incline gradually changes, shavings that are produced by the corner cutting edges 4 are discharged in an elongated form along these rake faces, and the problem also arises that there is a deterioration in the ability to process these chips.
Furthermore, FIG. 32 is an enlarged view showing principal portions of this conventional radius end mill. Corner portions 6, which are convex on the corner cutting edge 4 side, are formed on rake faces 2A and 4A as a result of an inner edge 2B (i.e., a boundary line between the rake face 2A and a wall face that protrudes outwards on the front side in the rotation direction T of the tool from the rake face 2A) of the rake face 2A of the end cutting edge 2 and an inner edge 4B (i.e., a boundary line between the rake face 4A and a wall face that protrudes outwards on the front side in the rotation direction T of the tool from the rake face 4A) of the rake face 4A of the corner cutting edge 4 intersecting at an obtuse angle.
However, in this type of radius end mill, a shortening of the interval from the end cutting edges 2 and corner cutting edges 4 to the inner edges 2B and 4B that corresponds to the size of the corner portions 6, which are intersecting portions between the inner edges 2B of the rake faces 2A of the end cutting edges 2 and the inner edges 4B of the rake faces 4A of the corner cutting edges 4, may not be avoided. In conjunction with this, because it also becomes impossible to ensure a sufficiently large space for the discharging chips, the problem arises that there is deterioration in the ability to discharge the chips.
In particular, in a radius end mill in which a ratio r/D between a radius of curvature “r” of substantially arc-shaped portions formed by the corner cutting edges 4, which constitute the intersection portions (i.e., corner portions) between the peripheral cutting edges 3 and the end cutting edges 2, and a diameter D of the tool body 1 is set to 0.2 or more, or in a radius end mill in which the radius of curvature “r” of the substantially arc-shaped portions formed by the corner cutting edges 4 is set to (D−d)/2 or more with respect to the diameter D and the web thickness “d” of the tool body 1, because the corner cutting edges 4 are enlarged and there is a tendency for the interval from the end cutting edges 2 and corner cutting edges 4 to the inner edges 2B and 4B to be reduced, the above described problem of there being a deterioration in the ability to discharge the chips is conspicuous.
Moreover, in the corner portions 6 in which the inner edges 2B and 4B intersect with each other, the chips easily become caught up and the presence of the corner portions 6 causes a further deterioration in the ability to discharge the chips.